On the morning of March 11 2004, as thousands of commuters made their way to work, 10 bombs packed with nails and dynamite exploded on four trains heading into central Madrid. The blasts killed 191 people and injured nearly 1,800. It was the worst Islamist terrorist attack in European history.

Clara Escribano, who was travelling to work when her train was torn apart in one of the attacks, still lives with the memory. "I have a film of that day constantly playing in my head," she said. "I still haven't been able to get on a train. In fact, I cannot even walk on the same side of the road as the station where I got on the train."

The events of 11-M, as the attacks are known in Spain, initially divided the country along political lines. The bombings were carried out just three days before a general election, which saw the incumbent conservative Popular party (PP) of José María Aznar defeated by the Socialist PSOE led by José Luis Rodríguez Zapatero.

From the moment the attacks took place, the PP argued that they were the work of the Basque separatist group Eta; Mr Aznar went so far as to phone national newspaper editors, assuring them this was the case. Despite evidence soon emerging of a van containing detonators linked to the attacks and a recording of verses from the Qur'an, the PP stuck to its line.

The police investigation and subsequent trial uncovered no evidence of a link to Eta. The bombings were carried out by a group of young men, mostly from north Africa, who were, according to prosecutors, inspired by a tract on an al-Qaida-affiliated website that called for attacks on Spain. The tract called for "two or three attacks ... to exploit the coming general elections in Spain in March 2004", saying that they would ensure the "victory of the Socialist party and the withdrawal of Spanish forces [from Iraq]".

Shortly after the PSOE's electoral victory and the confirmation of the withdrawal of Spanish troops, explosives were found next to the high-speed train line between Madrid and Seville. This raised further questions about the bombers' motives. Was the aim to remove Spanish troops from Iraq, because if that was the case, why plant a second bomb? Or was the target Spain itself? For many Islamist fundamentalists, there is a need to recover the former Muslim lands of "al-Andalus" from Spain, which was taken by Christian armies in 1492.

The bomb plotters were assisted by a gang of mainly Spanish small-time criminals who provided the dynamite needed for the attacks. The two groups - one religious, one criminal - came together through a series of coincidences and loose connections. An associate of the plotters had spent time in prison with one of the members of the criminal gang, so when the terrorists were looking for dynamite to purchase, he was able to put the two sides in contact. It remains unclear to what extent, if at all, the Spaniards knew about the Madrid plot.

After a long manhunt, the Spanish police surrounded a flat in Madrid three weeks after the bomb attacks, where seven of the suspected ringleaders were hiding out. But the terrorists had been tipped off by an alleged co-conspirator and as the police moved in, they blew themselves up, taking vital evidence with them. Among those who died were Serhane Ben Abdelmajid, the alleged mastermind behind the plot and known as the Tunisian, and Jamal Ahmidan, a hashish trafficker turned fundamentalist nicknamed the Chinaman.

At least four other suspects, including two who may have been central to the attack, have disappeared. One is understood to have died in a subsequent suicide attack in Iraq.

The figure that drew most attention at the subsequent trial was Rabei Osman, said to be the link between the Madrid bombers and other Islamist terrorist groups. Also known as the Egyptian, Osman was arrested in Milan in June 2004 after allegedly saying in wiretapped conversations that he planned the train bombings. Osman denied this, claiming he had been mistranslated, and condemned the attacks during the trial.

For victims, the politicisation of 11-M and the trial only made their suffering worse. "The political and media manipulation of the trial has been shameful, they [the politicians] have used the trial and the victims for their political games," said Ms Escribano.

The attacks took place shortly before the 2004 election, and the verdict has come out as Spain builds up to spring elections. Ms Escribano said she feared that politicians would "play with the victims all over again".

Rogelio Alonso, a lecturer in politics and terrorism at King Juan Carlos University, said he believed the trial had been a successful one. It had shown that "it is possible to fight this type of [Islamist] terrorism through the courts", he said, and that the investigation had uncovered a link between the Madrid suspects and the wider world of al-Qaida.

However, Scott Atran, an American academic who has investigated the Hamburg cell connected to the September 11 2001 attacks in the US and numerous other terrorist attacks around the world, witnessed much of the trial and described it as "a complete farce".

There has been much speculation in the press that the al-Qaida leadership was involved in some form in the Madrid attacks, but Mr Atran dismissed this, and the investigation provided no evidence to support it.

"There isn't the slightest bit of evidence of any operational relationship with al-Qaida," said Mr Atran. "We're been looking at it closely for years and we've been briefed by everybody under the sun and ... nothing connects them.

"The overwhelming majority of [terrorist cells] in Europe have nothing to do with al-Qaida other than a vague relationship of ideology. And even that ideology is fairly superficial - it's basically a reaction to what they see as a war on Islam around the world," he said.

But, argued Mr Atran, people needed to believe that something bigger was involved - it was hard to accept that a small, loosely connected group of young men could carry out an attack on this scale without outside assistance. "These young men radicalised themselves," he said.

For some victims, the trial has nonetheless been a cathartic experience. "I respect the work of the judge and the trial. I hope to get some kind of closure from the verdicts, and to be able to relax," Ms Escribano said.

Others fear that today's verdicts will not bring them the closure they need. Angeles Pedraza lost her 25-year-old daughter on March 11, and said she believed there were still some "important holes in the investigation" that she would like to be investigated, even if it meant further trials in the future.

Ms Pedraza alleged that the heads of the police and intelligence services might be "guilty of negligence, and we would like to see that investigated". Some key suspects had been followed by anti-terrorism officers from the beginning of 2003, but the surveillance team was taken off the case in February 2004. Some of the officers were detailed to work on Prince Felipe's wedding.

Ms Pedraza said questions also remained about "who financed the plot, and what other groups were involved".